The present invention relates to self-tapping concrete screws and, more particularly, to a screw which may be readily tapped into a pre-drilled bore in a concrete or other masonry structure without danger of stripping the thread and with improved embedment for a given depth of penetration.
The term "concrete" as used herein is intended to include aggregate materials, such as gravel, pebbles, sand or the like in a mortar or cement matrix, as well as masonry materials, such as stone, brick, concrete block and the like.
As is readily apparent to those skilled in the art, a self-tapping screw offers many advantages over other types of concrete anchoring devices in terms of both the installation time involved and the additional cost of producing a complex anchoring structure.
Those seeking the advantages of a self-tapping screw for concrete structures have been constrained by the fact that the concrete material tends to crumble, chip and break off from the wall of the pre-drilled bore as the thread cutting elements of the screw engage and cut into the bore wall. As a consequence, a substantial amount of concrete dust and particulate matter are produced in the bore. As the screw becomes further embedded into the concrete workpiece, more and more chips and dust accumulate between adjacent threads until a point is reached at which the torque required to achieve additional penetration of the screw is greater than the shearing or twist-off torque of the screw. Such problems can occur after only a few rotations of the screw and can result in a portion of the twisted-off screw shank becoming embedded below the surface of the concrete structure making it extremely difficult to remove without irreparably damaging the pre-drilled bore. Even if the screw is fully embedded in the bore, the threads that are formed in the bore wall are of such poor quality that the holding power, i.e., the pull-out strength, of the screw is significantly diminished.
It has also been discovered that there is a problem associated with the use in concrete aggregate material of conventional self-tapping concrete screws heretofore available. When a bore is pre-drilled into the concrete aggregate material, it frequently occurs that the drilled bore passes through or closely adjacent a piece of aggregate or gravel having a hardness greater than the hardness of the screw threads. If that occurs, the thread-cutting elements of the screw are usually damaged to such an extent that the thread cutting capability of the screw is virtually destroyed. Thus, even if the screw is driven into the bore, the pull-out strength will be completely unsatisfactory.
A conventional self-tapping concrete screw in widespread use in the industry is disclosed in U.S. Pat. No. 3,937,119. The screw disclosed in that patent has dual threads of different crest diameters, known in the art as "hi-lo" threads. The function of the thread convolutions with the greater crest diameter is to cut into the wall of the pre-drilled bore and to provide the holding power of the screw. The thread convolutions with the smaller crest diameter correspond roughly to the diameter of the borehole. The smaller diameter thread is said to guide the screw shank straight down the borehole and to provide reservoirs or spaces in which the dust produced by the cutting action of the screw can accumulate without causing undue stress in the concrete material or shearing of the screw.
It should be readily apparent that if half the threads of a concrete screw function solely as a guide means or to provide gaps or spaces for dust collection, only the remaining half of the threads function to penetrate or tap the bore wall and hold the screw in place. Thus, the embedment or pull-out strength of such prior art screws will be substantially less than a screw having the same number of threads, all of which penetrate or tap the bore wall.
A further shortcoming of the conventional self-tapping concrete screw disclosed in the aforementioned patent is the stringent tolerance requirements for the pre-drilled bore diameter and, consequently, for the drill bit diameter tolerance. In order for the smaller crest diameter thread, i.e., the "lo" thread, to function as a guide and not as a thread-cutting element, the pre-drilled bore diameter must exactly correspond with the smaller crest diameter. Variations from correspondence of those diameters will either reduce the guiding effectiveness of the "lo" thread or permit the "lo" thread to engage the bore wall and cause crumbling or chipping of the wall.
Another drawback associated with the aforementioned patented screw is the "hi" or cutting thread which comprises a sharp and relatively "thin" thread convolution as measured in the axial dimension of the screw at the root of the cutting thread. Such a "thin" thread coupled with a relatively low hardness of the screw thread can result in collapsed or severely damaged threads, especially when the screw is used in concrete materials with particularly hard aggregate.
Another way in which the problem of dust particle collection has been dealt with in the prior art is disclosed in U.S. Pat. No. 3,902,399 wherein thread convolutions of different crest diameters are provided at different longitudinal portions of the screw shank. An intermediate longitudinal portion of the screw shank is provided with threads with a crest diameter smaller than the bore diameter or with no threads at all. The space provided between the bore and screw shank at such intermediate portion is said to accommodate the dust formed as the screw penetrates the concrete. Such a thread arrangement likewise reduces the pull-out strength for a given length of thread.